The natural gas and oil exploit industry employs pipes having a fixed length which must be connected to each other's ends in order to reach the huge depths where hydrocarbon mines are generally located.
In the most commonly employed drilling technique, wells starting from the earth or the sea surface are drilled in order to reach the oil or gas field. Said wells can have a depth of several thousands meters. During drilling, said wells are coated inside with metal pipes for their entire length. The segments of metal pipe, which have a length of about 10 metres, are connected to each other by means of threaded joints. Therefore, these pipes form a tubular line, having the same diameter for its entire length but in the joints, whose external diameter is generally at least 1 inch (25.4 mm) bigger than the one of the line.
In order to form a coating for the entire depth of the well several lines are used having, due to mechanical resistance and to the geological characteristics of the site, the smaller diameter the bigger is the depth of the line, in order to form a “telescopic” structure. It follows that, since the diameter at the bottom of the well varies according to the pressure and the flow of the fluids to be extracted, the deeper the well, the bigger its diameter will be on the surface. As unfavourable consequence of the aforesaid, the drilling cost is high and, furthermore, these well coatings require a large amount of material, thus being very expensive. A smaller well diameter also means shorter drilling and coating times. Therefore, it is essential to minimize the well diameter and the coating pipe diameter with an equal amount of extracted fluids.
Once ended the drilling, inside the coated well it is inserted another tubular line, which must pump the gas or the oil out of the underground field. This line, which follows the entire depth of the well and can therefore reach a length of several thousands metres, is also formed by connecting pipes having a length of about 10 metres by means of the aforesaid joints. Usually, also this second kind of line has the same diameter for its entire length but in the joints, whose external diameter is generally bigger, as it happens in the previous case.
In both the aforesaid cases, the pipes are connected by means of threaded joints, which can be integral, and in this case an end of the pipe has a male threading and the other has a female threading, or can be of a coupling type, and in this case both ends of the pipe have a male threading, being connected by means of a female threaded coupling on both sides. Usually, the presence of the joint involves an increase of the external diameter of the line where the joints are; the bigger the external diameter of the joint if compared to the pipe, the bigger are the general dimensions of the line and the bore.
After the oil companies have required to minimize the oil and gas mining costs, remarkable efforts were carried out in order to reduce the diameter of the wells and therefore the diameter of the pipes.
In order to reduce the external diameter of the line and therefore the drilling costs and the amount of required material, they are used threaded joints having reduced diametrical dimensions, which can be divided into three different types, according to the performance required and to the maximum dimensions allowed. A first type, which is often called “semi-flush”, is a coupling joint whose external diameter does not exceed over 6% the pipe external diameter. A second type, usually called “near-flush”, is an integral joint whose external diameter is about 2–3% larger than the pipe external diameter. A last type, called “flush”, is an integral joint whose external diameter corresponds to the pipe external diameter.
The choice among the different types of joints depends on the load on the pipe line, the pressure acting internally and/or externally, its length and its maximum diametrical dimensions allowed with regard to the diameter of the well.
If the joint diameter is reduced it is necessary to find new solutions able to compensate for the lower structural resistance. In fact, in the parts close to the joints the efficacy is necessarily less than in the pipe, because the structural elements such as the threading, the seals and the shoulders are placed in the thickness of the pipe wall and this involves a reduced section in the critical parts of the male or female element.
A minimization of the pipe breaking causes is essential because said breaks, above all after the pipes have been laid in the underground, and it is therefore nearly impossible for the operators to replace a broken joint, can have extremely serious economical consequences and can cause environmental problems, particularly if the oil or gas field contains aggressive elements.
Therefore, in the past much has been done to improve joints and to reach an optimal efficacy level, balancing the different needs, which are sometimes in contrast among them, of minimum dimensions, maximum structural resistance and tightness against the output and/or input-of the fluids. In fact, the pipes undergo several forces, namely compression, traction, flexion and the pressure produced by external fluids and/or by fluids circulating inside the pipes.
The joints must also have an optimal resistance to screwing and seizing. Structural and sealing problems are often worsened by the fluid temperature, by their corrosiveness or by the environmental conditions of the digging areas. In the past several solutions have been proposed in order to satisfy the aforesaid conditions.
The international patent application WO-A-93/18329 concerns a joint with reduced diametrical dimensions having a central shoulder. Said shoulder is provided, both on the male element and on the female element, with a projection and a cavity which are parallel to the pipe axis, having homologous surfaces to be coupled in order to block the two joint elements. On the shoulder projections two sealing surfaces are provided. The shoulder divides two radially offset threading parts, which are cone-shaped or cone-shaped and cylindrical.
This joint is very efficient, but has a particularly complex structure involving very high production costs.
The European patent application EP-A-767335 concerns a joint with reduced diametrical dimensions provided with a central shoulder and two metal seals placed on the end of the male and the female elements. The shoulder divides two radially offset threading parts which are cone-shaped and cylindrical.
This joint's structure is simpler than the previous one, but it is always relatively complex because of the cone-shaped, cylindrical threading. Furthermore, the two seals at the ends of the joints hermetically seal its inside, thus pressurizing the lubricating fat for the screwing, with possible negative effects on the joint tightness.